1. Field of the Invention
The present invention relates to a method and apparatus by which lossless conveyance of user information can be achieved in an ATM network. The invention includes a protected bandwidth efficient network connection that can operate even using the bandwidth efficient 1:1 model while at the same time effecting a seamless (lossless or hitless) transition from the use of a working path to a protection path when a working path impairment occurs. A predetermined delay of the sending of information over the network is employed. Upon an indication of an impairment of service, the delayed information is sent over a second path.
2. Related Art
The International Telecommunications Unionxe2x80x94Telecommunication Standardization Sector""s current draft ITU-T I.630 Recommendation xe2x80x9cATM Protection Switchingxe2x80x9d has codified a set of four scenarios within which there are well defined specifications to assure high levels of, but not absolute continuity of, an ATM bearer service in spite of a wide variety of hard and soft OSI Physical layer impairments. Two pertinent inventions are known that have described the prior art pertaining to xe2x80x9cATM Protection Switching.xe2x80x9d
The first invention, entitled xe2x80x9cTransparent Non-disruptible ATM Networkxe2x80x9dby Dhadesugoor R. Vaman, Tai Noh and J. Bose, U.S. patent application Ser. No. 08/862,631, filed May 23, 1997, now U.S. Pat. No. 6,011,780, relates to providing a high level of assurance for user services carried over an ATM sub-network. This level of assurance is maintained in the face of link and intermediate ATM switch failures as well as traffic congestion conditions that occur at intermediate network nodes or links between nodes.
The second invention, entitled xe2x80x9cHigh Integrity Transport For Time Critical Multimedia Networking Applicationxe2x80x9dby Dhadesugoor R. Vaman, Cadathur V. Chakravarthy and Kicheon Hong, U.S. patent application Ser. No. 08/945,453, filed Oct. 24, 1997, now U.S. Pat. No. 6,079,042, and U.S. patent Ser. No. 08/901,825 filed Jul. 28, 1997, now U.S. Pat. No. 5,993,056, relates to a method of transport that corrects errored cells and replaces missing cells in an integrated fashion with minimal overhead in sending the correction data.
These patent applications teach that although a solid failure (link or node) may occur within a sub-network, the techniques described therein can restore the bearer services that are provided by the sub-network with various degrees of temporary loss of service. The entire disclosures of these patents are expressly incorporated herein by reference.
It has been proposed to duplicate a simplex node to node physical link coupled with a transmitter that generates cell flows that are sent simultaneously to two Physical Media Dependent (PMD) link drivers. At receiving PMDs the two flows are recovered and synchronized by using the properties of F3 OAM cells, normally used for maintenance and performance monitoring of the physical link. These cells are specific to Cell-Based physical layers and are inserted in the cell flow on a cyclically recurrent basis. The user information stream may thus be seen as being xe2x80x9cblockedxe2x80x9d by these regular synchronizing F3 OAM delimiter cells. Synchronization at the cell level is essential to compensate for the length difference between the two physical links. Once this is achieved protection of user service is performed by selecting cell by cell (or block by block) those cells (or whole correct blocks) that are determined to have been correctly delivered by one or other of the two links.
This proposal is intended to be used exclusively over a simplex point to point link (but this scheme could be duplicated to support a duplex link). The proposal does not include a method for xe2x80x9chitlessxe2x80x9d communication over an end to end connection within an ITU-T I.630DR ATM Protection Switching domain as does the present invention.
While much of what is taught in the aforementioned patent applications has been codified within the ITU-T Draft Recommendation I.630xe2x80x94hereafter identified as I.630DR, ITU-T SG13 recognizes a need to further extend ATM""s Protection Switching capabilities by an item in its xe2x80x9cLiving Listxe2x80x9d entitled xe2x80x9cHitless ATMxe2x80x9d indicating that this is a field for farther study. The term xe2x80x9cHitless ATMxe2x80x9d identifies a user service provision between connected source and destination nodes can be assured without even temporary loss of delivered information, despite the occurrence of various forms of bearer service impairments that affect ATM cell transit over any of the intermediate network connections.
A principal object of the present invention is the provision of an ATM network where data loss is prevented.
Another object of the present invention is the provision of a modified ATM network where loss of data is prevented on the occurrence of a fault in the transmission path from a transmitting node to a receiving node.
A still further object of the present invention is the provision of a delay in a data or information stream so that a delayed version of the data or information is always available in the event of a fault.
An additional object of the present invention is to provide a hitless ATM method wherein data is delayed and can be sent over a secondary path upon the indication of a network impairment.
Another object of the invention is the provision of a method and apparatus for assuring lossless delivery of data or information in both 1:1 and 1+1 network configurations.
The method and apparatus of the present invention is applicable in the scenario, identified as a xe2x80x9c1:1xe2x80x9d case where the xe2x80x9cprotectionxe2x80x9d or secondary path is not used until impairment in the working path is detected. Only then is it deemed appropriate to re-route the services carried by the working path onto the protection path to assure continued integrity of the user services provided.
The method and apparatus of the present invention is also applicable to the scenario identified as a xe2x80x9c1+1xe2x80x9d case where the source node transmits the user service concurrently over both the working and protection paths. When such a dual working and protection path connection is established only the information on the path designated as xe2x80x9cworkingxe2x80x9d is used at the destination node. However, when an impairment in the working path is detected the destination node switches to the use of the protection path information concurrently relayed to it thereby overcomes the absence or impairment of the working path in the user bearer service connection.
The present invention uses a point to point link only based hitless mechanism. The synchronization process is employed end to end across an I.630DR like protection domain. A xe2x80x9cqueuexe2x80x9d is added into the user information stream at the source node of the protection domain, and a working path receive queue is also introduced at the destination node. Finally a low bandwidth sequence numbered (F3 like) OAM cell stream is carried cyclically from the source to the destination node over both the working and protection paths.